Remove vias if they only connect to single layer

A via takes up zone space on additional layers, so if we are not
connecting to those layers, then we should remove it.

Fixes https://gitlab.com/kicad/code/kicad/issues/7004

pcbnew/connectivity/connectivity_data.cpp

@@ -612,6 +612,27 @@ bool CONNECTIVITY_DATA::TestTrackEndpointDangling( TRACK* aTrack, wxPoint* aPos
         }
     }
 
+    // Test if a via is only connected on one layer
+    if( aTrack->Type() == PCB_VIA_T )
+    {
+        const CN_ITEM::CONNECTED_ITEMS& connected = citem->ConnectedItems();
+
+        // This is a bit redundant but better safe than sorry here
+        if( connected.empty() )
+            return true;
+
+        // Here, we check if the via is connected only to items on a single layer
+        int first_layer = connected.front()->Layer();
+
+        for( auto& item : connected )
+        {
+           if( item->Layer() != first_layer )
+               return false;
+        }
+
+        return true;
+    }
+
     return false;
 }